CVE-2026-49840: FreeSWITCH: Pre-authentication heap buffer overflow in libesl `Content-Length` parsing

Vulnerability summary

CVE-2026-49840 is a critical (CVSS 9.1) Improper Input Validation vulnerability in freeswitch. EPSS estimates a 0.31% chance of exploitation in the next 30 days, higher than 24% of scored vulnerabilities.

Timeline

1. 09-06-2026
   PublishedVulnerability first disclosed.
2. 09-06-2026
   Last modifiedVulnerability information updated.

Description

FreeSWITCH is a Software Defined Telecom Stack enabling the digital transformation from proprietary telecom switches to a software implementation that runs on any commodity hardware. Prior to version 1.11.1, esl_recv_event() parses Content-Length with atol() and passes the result straight to malloc(len + 1) with no sign or magnitude check. A malicious or man-in-the-middle ESL peer can send a frame with a negative Content-Length to corrupt the heap of, or crash, any process linked against libesl, before the client has authenticated to that peer. This issue has been patched in version 1.11.1.

• CWE-20Improper Input ValidationThe product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
• CWE-122Heap-based Buffer OverflowThe product writes past the end of a buffer that was allocated on the heap.
• CWE-195Signed to Unsigned Conversion ErrorThe product uses a signed primitive and performs a cast to an unsigned primitive, which can produce an unexpected value if the value of the signed primitive can not be represented using an unsigned primitive.
• CWE-787Out-of-bounds WriteThe product writes data past the end, or before the beginning, of the intended buffer.